1. Field of the Invention
The present invention pertains to a rotating anode for X-ray tubes, an anode of a composite type with graphite.
The X-radiation produced by an X-ray tube results from the bombardment, by a cathode-generated beam of electrons, of a refractory material with a high atomic number borne by the anode. These materials which possess a high atomic number comprise, for example, tungsten, tantalum and, also, molybdenum: these materials are called "target materials" in the rest of the description.
The emission of X photons is accompanied by a high emission of heat: the energy yield of the X rays produced, i.e. the ratio of the energy of the X photons to the energy of the impinging electrons is about 1% and the rest is transformed into heat.
In general, it is only by radiation that the heat accumulated in the anode is discharged. Hence anodes, and especially rotating anodes, are most often built so as to favour heat radiation and, to this effect, they comprise one or more graphite elements.
The essential function of the graphite is to increase the thermal radiation. The increase in thermal radiation .DELTA.W can be written: ##EQU1## where W is the energy and .epsilon. is the coefficient of radiation or the coefficient of emissivity. The gain in dissipated energy varies linearly with the coefficient of emissivity, moreover when all conditions are equal.
Besides, the energy radiated W is proportionate to the temperature to the power of 4, expressed in .degree.K. Thus, for a radiated energy W1 corresponding to a temperature T1=1250.degree. C., and a second radiated energy W2 corresponding to a second temperature T2=1000.degree. C., the ratio ##EQU2##
This shows how important it is to be able to carry the anode disk to the highest possible temperature so as to derive the maximum advantage that can be given by the thermal radiation due to graphite.
The contribution of graphite in the rotating anode disks can be provided in different ways. As a general rule, the anode disk is a composite disk formed of a basic body, one surface of which at least partially lined with a target material.
The basic body may be directly made of graphite. The target material, tungsten for example, can be applied to the graphite either by brazing processes or, for example, as a layer deposited on the graphite through a depositing process by gaseous-phase depositing or, again, by electrolysis in the dry way. In any case, the quality of the bond between the tungsten and the graphite is of prime importance, on the one hand to obtain adequate adhesion of the tungsten to the graphite and, on the other hand, to establish a minimum thermal resistance between the tungsten or other target material considered to be the source of the heat, and the graphite which is provided to discharge the heat by radiation.
This bond between the tungsten or other target material and the graphite is made by a layer of bonding element: in the case of brazing, it is the brazing element which constitutes this bonding element and, in the case of gaseous-phase depositing or electrolysis in the dry way, this bonding element is made up of a so-called intermediate element, deposited as a film between the target material and the graphite: this intermediate element is generally made of rhenium, which itself is a refractory material.
In other cases, the basic body is made, for example, of molybdenum to which the target material, such as tungsten, is applied according to a mechanical process for example: a graphite element is brazed to the basic body made of molybdenum, to a surface opposite to that of the target material. The quality of the bond between the molybdenum and the graphite is as important as in the previous examples, the bonding element being made of a relatively refractory material such as, for example, zirconium, titanium, palladium, rhodium etc.
Whatever the composition of these composite-type rotating anodes, it is frequently observed that the differences in temperature between the target material and the graphite element are greater than expected and that, consequently, the quantity of energy radiated is considerable less than hoped for.
The author of the present invention believes that this defect is due to the unsatisfactory quality of the graphite-tungsten or graphite-molybdenum bond and that, especially as regards the brazing processes, the brazing elements referred to above inadequately wet the graphite as well as the tungsten or molybdenum.
Furthermore, it must also be observed that if the brazing element has an excessively low melting point or an excessively high vapour point, these factors can lead to a reduction in the working temperature of the entire anode disk and can thus result in a diminishing of the quantity of energy radiated.